1. Field of the Invention
The present invention relates to a testing protocol for infant lung function.
2. Brief Description of the Related Art
Lung function testing (LFT) is an established mostly standardized routine diagnostic modality in clinical medicine for older cooperative children and adults but not infants. Since infants cannot cooperate, infant lung function testing (ILFT) is more tedious and lengthy to accomplish. It is operator, technique(s), hardware and software-dependent and is performed with the infant lying supine using a facemask during a limited unpredictable period of a chloral hydrate-induced sleep. The drug use precludes repeat or frequent testing.
Therefore, ILFT has for many years often been confined to relatively insensitive tests within the normal tidal breathing range.
Despite being an unreliable volume landmark in infants and young children, the functional residual capacity (FRC), which is the residual gas in the lungs and airways and represents the oxygen stores at end-tidal expiration, has until recently been the only lung volume to be routinely and reliably measured in this inherently uncooperative age-group. FRC measurement is primarily important for defining the normal lung growth and development, in assessing longitudinally a suspected impairment of alveolar growth and for interpreting lung mechanics and tidal expiratory flows.
With normal tidal breathing in adults and older children the normal end-expiratory lung volume, that is the static or passive FRC coincides with the elastic equilibrium volume (EEV) of the respiratory system where the outward recoil of the chest wall is balanced by the inward recoil of the lungs. In infancy, the compliance of the chest wall is nearly threefold that of the lung. By the second year of life, the increase in chest wall stiffness is such that the child's chest wall and lung have similar compliances, as in adults. Hence, in infants, the outward recoil of the chest wall is very small and the inward recoil of the lung slightly less than in adults. Therefore, the balance of the elastic recoil forces of the lung and chest wall in the infant predicts a very low FRC of only 10% of the total lung capacity (TLC), which is incompatible with the appropriate stability of the peripheral airways or adequate gas exchange. Therefore, infants incorporate breathing strategies which include postinspiratory activity of the diaphragm, laryngeal narrowing during inspiration and braking of expiration to maintain a dynamic FRC (dFRC) above the passively determined level, that is the static (sFRC) or passive FRC, and inspire before expiration ends passively. Compared to adults, infants terminate expiration at substantial flow rates. This breathing pattern results in a substantial increase in an end-expiratory lung volume above the passive level and a dynamic FRC/TLC ratio of 40% comparable to that of a supine adult. Taken together, any routine measurement of FRC in spontaneously breathing infants has always been a measurement of the dynamic FRC (dFRC) and the variability in end-expiratory level has impeded the assessment and interpretation not only of lung volumes but also of respiratory mechanics and forced expiratory flows.
Forced expiration (FE) is now widely generated with the rapid thoracoabdominal compression (RTC) technique using a squeeze jacket from a raised lung volume (RVRTC) mostly to an airway opening pressure (Pao) of 30 cm H2O which generates forced expiratory flow-volume (FEFV) curves in which flow limitation is better achieved. Nevertheless, the RVRTC technique remains fairly complex and difficult to perform and has neither been standardized nor its clinical utility established. Subtle changes and differences in methodology between various laboratories could lead to significant variations in the shape and smoothness of the FEFV curves generated and the instant or forced expiratory flows (FEF%) have been less reproducible than the volume-time (FEVt) variables. With the rapid somatic growth and development in infants and changes in clinical status in those with disease, an impeccably high degree of repeatability and accuracy of simultaneous measurements of lung volume and airway function is essential in order to detect and quantify the earliest pathophysiological changes. It is especially significant that repeatability data on the same infant is lacking. The squeeze jacket placement and the need to repeat the RVRTC using increasing jacket pressures (Pj) until flow limitation is achieved may alter lung mechanics or influence subsequent measurements and other variables.